Well boreholes are typically drilled in earth formations to produce fluids from one or more of the penetrated formations. The fluids include water, and hydrocarbons such as oil and gas. Well boreholes are also drilled in earth formations to dispose waste fluids in selected formations penetrated by the borehole. The boreholes are typically lined with a tubular commonly referred to as “casing”. Casing is typically steel, although other metals and composites such as fiberglass can be used. The outer surface of the casing and the borehole wall form an annulus, which is typically filled with a grouting material such as cement. The casing and cement sheath perform several functions. One function is to provide mechanical support for the borehole and thereby prevent the borehole from collapsing. Another function is to provide hydraulic isolation between formations penetrated by the borehole. The casing can also be used for other functions such as means for conveying borehole valves, packers, pumps, monitoring equipment and the like.
A variety of acoustic borehole logging systems are used to evaluate casing condition and the effectiveness of hydraulic isolation provided by cement filling the casing-borehole annulus. The Ultrasonic Radial Scanner logging system disclosed in U.S. Patent Application Publication No. US 2006/0067162 A1 is particularly effective in evaluating casing and the near borehole environs. Among other parameters, the Ultrasonic Radial Scanner tool measures the acoustic impedance of material within the casing-borehole annulus.
During a typical acoustic impedance logging operation, the logging tool is located at an axial position in the borehole where there is little or no cement is the casing-borehole annulus. This condition is commonly referred to as “free pipe”. The density of drilling fluid used in drilling the well is typically known. Drilling fluid acoustic impedance can be estimated in free pipe intervals using drilling fluid density and the type of drilling fluid (e.g. oil based or water based) using empirical relationships known in the art. An estimated “acoustic impedance calibration” value is then obtained within the borehole by combining the response of the logging tool in free pipe with estimates of drilling fluid acoustic impedance.
Potential error is introduced in the above in situ acoustic impedance calibration method from several sources. As a first example, there is no assurance that the density of drilling fluid in the casing-borehole annulus is the same as the density of the drilling fluid used to drill the well. Density can change due to contamination of formation fluids, particulate material from the drilling fluid, and the like. As a second example, the type of fluid in the casing-borehole annulus can change during the drilling operation. Formation water can commingle with oil based drilling fluid, or formation water can commingle with water based drilling fluid. As a third example, the relationship between drilling fluid density and drilling fluid acoustic impedance is empirical thereby introducing another potential source of error.
There is also no assurance that an axial interval of free pipe is present in the cased borehole. This negates the use of the in situ acoustic impedance calibration discussed above. The logging system can alternately be calibrated at the surface of the earth using calibration fixtures comprising casing of known dimensions and acoustic characteristics backed by materials of known acoustic impedance properties. The calibration is erroneous if the actual well casing does not match, in dimensions and acoustic properties, the casing of the surface calibration fixture. Furthermore the calibration is erroneous if the fluid within the casing-borehole annulus (which is typically unknown) does not match the acoustic properties of the material backing the casing in the surface calibration fixture.
Borehole environments are typically harsh in temperature, pressure and ruggosity, and can adversely affect acoustic transducer response. More specifically, acoustic measures of the borehole parameters of interest can be adversely affected by harsh borehole conditions. Changes in borehole temperature and pressure are typically not predictable. Acoustic impedance calibration within an axial interval of free pipe, or calibration in surface calibration fixtures, are not valid under continuously changing conditions encountered as the logging tool is conveyed along the well borehole.
In view of the above discussion, it is apparent that apparatus and methods for automatic, continuous and real-time calibrating a transducer of an acoustic logging system would yield more accurate measures of various properties of the borehole environs.